1. Field of the Invention
The invention relates to a heating device of the light irradiation type in which a semiconductor wafer (hereinafter called a wafer) is heated by light to form layers, for diffusion, annealing and the like.
2. Description of the Related Art
Heat treatment of the light irradiation type in the production of semiconductors is preformed in wide areas, such as in forming layers, diffusion, baking and the like.
In each of these treatments, a wafer is heated to a high temperature and treated. If, for this purpose, a heating device of the light irradiation type is used, the wafer can be quickly heated. The temperature of the wafer can be increased to 1000.degree. C. or greater after ten and a few seconds to a few dozen seconds. Furthermore, rapid cooling can be achieved when light irradiation is stopped.
A heating device of the light irradiation type is described, for example, in Japanese patent disclosure document HEI 8-45863. FIG. 1 shows this heating device. In the figure, a light irradiation chamber 1 is shown in which a wafer is seated on a wafer holding plate 2, heated and irradiated. The light source part for heating this wafer 3 has an arrangement in which several ring-shaped IR lamps 4, each with a different diameter, are arranged concentrically. A mirror 5 which is made of a metal, such as aluminum or the like, is provided with concentric grooves and with opening 6 through which insertion portion 8 of the lamps 4 pass. The shape of the lamp 4 is such that the lamp 4 fits into the groove. The reflection surface of the mirror 5 is galvanized with a metal which, during operation of the lamps 4, reflects the IR radiation emitted from the lamp 4, for example, gold. A fused silica glass window 9 is used to close off the chamber 1 relative to the lamps 4 when the atmosphere in the vicinity of the wafer 3 is different from the atmosphere in the vicinity of the lamps 4.
Due to the light from the lamps 4, the mirror 5 is caused to have a high temperature. Therefore, a cooling is provided according to the material of the mirror 5 and the heat resistance temperature of galvanization of the surface. If air cooling alone is not adequate, there is a water cooling device or the like. For example, in the case in which the mirror is made of aluminum the aluminum melts at temperatures of 600.degree. C. or greater. Here, water cooling is provided.
FIG. 2 shows an example of a lamp 4 which is used for a light source. The lamp has an arc tube portion 7 in which a light emitting filament 12 is located, insertion portion 8 which are located on the ends of the arc tube portion 7, and sealing areas 11 by which the ends of the insertion portion 8 are hermetically sealed and which have sealing foils through which the filaments 12 and power supply lines 19 are joined to one another.
As shown in FIG. 3, the sealing areas 11 and the insertion portion 8 of the lamp 4 pass through the opening 6 of the mirror 5. The arc tube portion 7 is located in the concentric groove of the mirror 5 and the lamp 4 is attached by means of an attachment component (not shown). As is shown in FIG. 1, the sealing areas 11 project upwardly out of the mirror 5. The lines 19 from the sealing areas 11 are connected to a lamp power source via connectors, terminal devices or the like.
FIG. 4 is a schematic in which the light source part which is provided with lamps 4 is shown from the side facing the wafer in FIG. 1. The cross-hatched area represents the through opening 6 of the mirror 5 through which the insertion tube 8 of the lamp 4 passes. Reference number 7 indicates the arc tube portion and 12 the filament in the arc tube.
In this conventional device, it is necessary to conduct cooling in such a way that the respective part of the lamp 4 reaches a suitable temperature. However, this point has not been adequately considered.
This means that, during operation of lamps, the respective part must be kept at a suitable temperature in consideration of the following:
1. The surface temperature of the bulb of the arc tube portion 7 in which the filament 12 emits light must be no greater than 800.degree. C. If the input power of the lamp 4 is increased for treatment of the wafer 3, the amount of light emitted from the filaments 12 is increased. In this way, the temperature of the bulb of the lamp 4 is increased. At a bulb temperature at least equal to 800.degree. C., the fused silica glass comprising the bulb recrystallizes, and thus milky opacification occurs; this is called "devitrification." When devitrification occurs, the transmission factor of the light decreases and the lamp 4 can no longer deliver a stipulated radiant energy to the wafer 3. PA0 2. The sealing area 11 must have a temperature no greater than 300.degree. C. At a higher temperature, the sealing foil (molybdenum foil) is oxidized and expanded. This causes cracks in the sealing area 11 and the sealing area 11 is damaged. PA0 3. The insertion tube 8 must have a temperature of at least equal to 250.degree. C. At a lower temperature, a tungsten-halogen compound condenses and accumulates on the inside wall of the insertion tube 8 which has a low temperature. Here, the tungsten-halogen compound consists of the halogen gas which fills the lamp 4 and the tungsten which has vaporized from the filament 12. The halogen cycle in which the vaporized tungsten is converted into a tungsten-halogen compound and returns again to the filament 12 therefore no longer exists; this causes dilution and burning through of the filament 12. The service life of the filament 12 therefore becomes shorter. Since the vaporized halogen gas diminishes, the phenomenon occurs that the vaporized tungsten is deposed on the inner wall of the arc tube portion 7 without reaction with the halogen. When blackening occurs, the radiant energy from the filament 12 is absorbed by the blackened area. The wafer 3 can therefore no longer be irradiated with a stipulated energy. The insertion tube 8 must therefore have a temperature of at least equal to 250.degree. C. Since the insertion tube 8 is an area which does not contribute to light irradiation of the wafer 3 and is not provided with an emission part of the filament 12, the temperature in this area often becomes low. It is therefore necessary to control the temperature of the insertion tube 8 to a suitable temperature. PA0 4. The heat treatment device for the wafer 3 must heat the wafer to a temperature of 800 to 1200.degree. C. Recently, an oxide layer has been produced in general at 1150.degree. C. When the lamp 4 is operated using the device shown in FIG. 1, no cooling of the lamp 4 is accomplished. To keep the temperature of the bulb of the arc tube portion 7 of the lamp 4 less than or equal to 800.degree. C., the maximum power per unit of length of the filament which can be supplied to the lamp 4 is 60 W/cm; this is not adequate for heating of the wafer 3 to 800 to 1200.degree. C.
To eliminate the above described defect it can also be imagined that the lamp input power can be increased and cooling air blown onto the lamp 4 at the same time to cool it. However, here only the temperature of the area of the lamp 4 on which cooling air is blown (the side opposite the mirror 5) is reduced, it being difficult for the cooling air to penetrate into the gap between the mirror 5 and the lamp 4. Therefore, it is difficult to cool this area (the area opposite the mirror 5). Furthermore, in addition to temperature control of the arc tube portion 7, it is necessary to keep the sealing area 11 and the insertion tube 8 in the above described temperature range. As a result, in each part, there must be several temperature control devices, causing the disadvantage that the device has a complicated arrangement.